[skrifa] autohint: schedule a cleanup pass

[dfrg]

Initial implementation was focused on matching FreeType both for expediency and to ensure correctness. There is a lot of room for improvement in code quality by making use of more idiomatic Rust code and adding methods/types for common patterns.

For example: https://github.com/googlefonts/fontations/pull/1134#discussion_r1747416382 and https://github.com/googlefonts/fontations/pull/1134#discussion_r1747417711

Also try to document more of the magic numbers. ref https://github.com/googlefonts/fontations/pull/1145#discussion_r1751401758